This study evaluated yearling growth and reproductive performance of Hereford heifers at 24-26 months of age grazing on natural pastures at two stocking rates during winter and spring, associated or not to improved natural pastures. Heifers were classified according to three weight groups (Light, Medium and Heavy) and randomly distributed into four feeding management practices: 0.6 AU (1 animal unit = 450-kg body weight) – heifers on natural pastures at a stocking rate of 0.6 AU/ha; 0.8 AU – heifers on natural pastures at a stocking rate of 0.8 AU/ha; 0.6 AUI – heifers on natural pastures at a stocking rate of 0.6 AU/ha and on improved natural pastures between Sept 1st and Nov 1st; 0.8 AUI – heifers on natural pasture at a stocking rate of 0.8 AU/ha and on improved natural pastures between Sept 1st and Nov 1st. The stocking rate of 0.8 AU resulted in significant weight loss during winter months (-0.138 and -0.356 kg for 0.6 and 0.8 AU, respectively). Heifers grazing during the spring on improved natural pastures presented higher weight gain before the breeding season and higher body weight and body condition score in the beginning and end of the breeding season in relation to those grazing only on natural pastures. Heifers kept at 0.6 AU/ha obtained 69.7% pregnancy rate, which was significantly higher than the 54.2% of those managed at 0.8 UA/ha. Heifers grazing on improved natural pastures during the spring achieved 83.8% pregnancy rate, whereas those maintained on natural pastures achieved only 40.2%. Light and medium heifers lost less weight during the winter as compared with the heavy ones, but weight differences between groups remained until the end of the breeding season. Heavy heifers have higher pregnancy rates. Heifers submitted to lower stocking rate and those grazing on improved natural pastures conceive earlier.